Consider working in the field as a teacher or counselor before applying for a doctoral degree in Gifted Education. You can make the transition directly from undergraduate without work experience, but many programs favor applicants who have taken at least a year or two off to gain practical insight into the field.
Familiarize yourself with the current leading researchers in the field and their work. This will help you to have a better grasp of what issue or problem you would like to be the focus of your graduate work.
Apply to programs that give you access to scholars with whom you're interested in working. In general, a program is only good for you if the department's research interests closely match your own, regardless of the school's overall academic strength.
Look for programs that have ties with the National Research Center for the Gifted and Talented. This is currently the foremost professional organization in the field and produces most of the good scholarship on issues in Gifted Education.
Pick a dissertation topic about which you're truly interested. One of the largest pitfalls for any doctoral student is choosing to write a dissertation or thesis on a topic that isn't really interesting, despite how important it may be to the field.